Certain acrylate radiation curable coatings and films are in great demand because of their rapid curing properties.
Since acrylates are not conducive to cationically induced radiation curing, they require free radical systems which are oxygen inhibited unless effected in an inert atmosphere, generally under a blanket of nitrogen. Although formulation with a photoinitiator which undergoes bimolecular reaction with a hydrogen donor minimizes the inhibitory effect of air, this benefit is realized at the expense of a greatly reduced curing rate. Also, it has been found that polymerization or curing in free radical systems ceases almost immediately upon removal from the source of radiation; thus, the cured product often contains significant amounts of unpolymerized components. The formulation of radiation curable coatings and films normally involve compositions containing acrylate monomers or acrylate functional oligomers together with free radical initiators and minor amounts of one or more adjuvants such as surfactants, slipping agents, defoamers, thickeners and thixothopes depending on the option of the formulator and consistent with the end use. Oligomers usually employed are acrylate functional end capped polymers such as urethane polyesters, polyols, acrylates, bisphenol A, epoxides and novalac epoxy resins.
Additional shortcomings of acrylate formulations includes their stability. Under normal storage conditions, these coating formulations require the incorporation of a free radical scavanger, e.g. a substituted hydroquinone or phenothiazine. However, after the coating has been applied, the inhibitory affect of the stabilizer contributes to a significantly reduced cure rate. Accordingly, it is an aim of research to develop oligomers which provide stable formulations and which have the beneficial properties of acrylates but which are amenable to radiation curing at a rapid rate by cationically induced polymerization which is not oxygen inhibited and which permits continued polymerization after removal from the source of radiation exposure.
It is also noted that the unsubstituted acrylates are sensitizers and skin irritants as well as being carcinogenic, so that specialized safety precautions must be taken to protect operators from exposure when these chemicals are handled in high concentration. Although alkoxylation has lessened skin irritancy of the acrylates, their carcinogenic properties are not reduced.
Some of the inherent deficiencies of the acrylate systems can be partially overcome by the use of epoxy resins. Epoxy resins can be polymerized by normal radiation techniques using cationic photoinitiators such as iodonium, sulfonium and ferrocene salts of hexafluorophosphate, hexafluoroantimonate and hexafluoroarsonate to produce a tack free film. In such formulations substantially complete polymerization of the mixture requires an extended period. It is well known that the polymerization of epoxy resins is extremely slow and requires as much as several days to achieve their ultimate physical properties. Thus, thermal curing is often employed to increase the rate of, or to complete the polymerization.
Accordingly it is an object of the present invention to overcome the above described deficiencies by employing an economical and commercially acceptable composition and curing process.
Another object of this invention is to utilize a multifunctional polymer, which is itself a polymerizable viscous liquid and which provides rapid radiation curing.
Another object is to provide a stable, non-toxic cross linkable copolymeric resin composition which is rapidly cured by cationic radiation to produce a film or coating on a substrate which possesses toughness, good adhesion, abrasion resistance and resistance to chemical attack in acid or basic media.
Still another object is to provide a more economical process for cross-linking monomeric or polymeric vinyl ethers which can be effected in the presence of air at ambient temperature and pressure.
Another object is to provide a vinyl ether, acrylate or epoxide composition which is curable at a rapid rate by radiation induced cationic polymerization.
These and other objects will become apparent from the following description and disclosure.